1. Field of the Invention
The present invention relates to a lens assembly for an optical instrument such as a camera. More specifically, the present invention is directed to an improvement in the structure of a helicoidal screw adapted for driving an objective lens in the direction of an optical axis.
2. Related Background Art
In an optical instrument with an objective lens assembly there is often produced the problem of harmful stray light. The stray light generated within the lens tube has various undesirable effects on the performance of the optical instrument.
A typical solution to the problem is to prevent the reflection of light on the inside circumferential surface of the lens tube. For example, in the case of a conventional picture-taking lens assembly comprising a plural lens groups designed for a camera, the inside circumferential surface of the lens tube is usually coated with a light-absorbing black paint. This black coating serves to prevent any scattered stray light from entering the film plane of the camera.
As well-known to those skilled in the art, a lens tube of a conventional camera lens assembly is usually composed of two tubes, namely an inner tube and an outer tube. Lenses are held in the inner tube and moved in a direction along the optical axis for focusing or zooming. To this end, the inner tube is provided with a male helicoidal screw formed on the outside circumferential surface. The male helicoidal screw is in mesh with a female helicoidal screw formed on the inside circumferential surface of the outer tube.
FIG. 1 shows an example of such a structure in a lens assembly for a camera.
Referring to FIG. 1, a lens tube is composed of an inner tube 8 and an outer tube 3. The inner tube 8 holds a lens 7 and has a helicoidal screw 8a formed on its outside circumferential surface. The outer tube 3 has a helicoidal screw 9 formed on its inside circumferential surface. The helicoidal screws 8a and 9 are in mesh with each other to couple the inner tube 8 with the outer tube 3.
The inner tube 8 is allowed to move in an axial direction only and its rotation relative to the camera body 1 is inhibited by stop means not shown.
The outer tube 3 is engaged in a stationary portion 1a of the camera body 1 through a screw 2. The outer tube 3 has also a gear 4 formed on the outside surface of the tube. The gear 4 is rotationally driven by a driving gear 5 which is in turn driven by a motor 6.
When the driving gear 5 is driven into rotation by the motor 6, the outer tube 3 is also rotated through the gear 4. While rotating, the outer tube 3 moves axially in accordance with the pitch of the screw 2. As the result, the inner tube 8 is moved together with the lens 7 linearly along the optical axis to attain focusing.
In the above-shown lens assembly, the problem of harmful stray light arises from the fact that the helicoidal screw 9 formed on the inside surface of the outer tube 3 is exposed to a beam of photo-taking light from the lens 7. As seen in FIG. 1, a portion of the photo-taking light from the lens 7 runs in a direction away from the optical path of the photo-taking light as represented by a beam A.sub.1. The off-optical path beam A.sub.1 falls on the helicoidal screw 9 and is reflected by it toward a film plane 13. As stray light, the reflected beam A.sub.2 enters the film and has an undesirable effect.
The effect of the harmful stray light is larger and more significant when the inner tube 8 is moved out toward the object side. This is because the number of exposed ridges of the helicoidal screw 9 is increased when the inner tube 8 is moved out.
FIG. 2 is a section view of a prior art helicoidal screw showing the conventional form of the screw.
As seen from the figure, the sectional form of the prior art helicoidal screw is trapezoidal. The ridge of the screw has a flattened apex 10. The flat portion 10 tends to reflect the off-optical path beam A.sub.1. The reflected light A.sub.2 passes through an aperture 12 (FIG. 1) and reaches the image-forming plane or film plane 13. Thus, the reflected light A.sub.2 constitutes harmful stray light which degrades the image-forming performance of the lens.